NATURAL PHILOSOPHY. 157 



The tremor produced by steam-power is more likely to affect the plates in 

 some parts of the ship. It is evident that there are causes in action tend- 

 ing to produce effects like those exhibited in Dr. Scoresby's experiments, 

 and it is equally evident that the action of those causes must be exceed- 

 ingly slow. On one point, however, I trust that a consideration of Dr. 

 Scoresby's experiments will disabuse many persons who have not been 

 well acquainted with the nature of induction and sub-permanent magnet- 

 ism. The change to be expected in a ship's sub-permanent magnetism, in. 

 sailing from England to the Cape of Good Hope, does not essentially 

 depend on her passing into another magnetic hemisphere. It does depend 

 mainly on this circumstance : that, supposing her to have been built with 

 her head to the north, or in the line of boreal magnetism, she is then 

 turned with her head to the south, or in the line of austral magnetism, 

 and is so kept, exposed to slight tremors, for one or more months. If she 

 had been moored off the coast of Portugal for the same time,^m the same 

 position, and exposed to the same tremors, I apprehend that her magnetism 

 would have undergone nearly the same change (as regards horizontal 

 deviation of the compass) as in the voyage to the Cape of Good Hope* 



4. I think the selection of the loss of the Tayleur, as the text for the 

 principal discussion on iron ships, with all its attendant horrors, (having 

 no application whatever to the matter under discussion,) was unfortunate. 

 When the feelings are excited, the judgment of the speaker, as well as of 

 the hearers, is very liable to be perverted. The question at issue is the 

 very abstract one : Is it likely that in two days the magnetism of a ship 

 could be so mush changed that the compass would be disturbed through 

 an angle of t\yo points? I unhesitatingly answer: It is not likely ; and, 

 speaking with our present knowledge on the subject, it is not possible. I 

 have already stated, that I conceive the causes pointed out by Dr. Scores- 

 by to be wholly inadequate to produce such a rapid change. And I aver, 

 that there is no known, instance of such a change ; and I do not believe 

 that an instance can be produced of a rapid change of one-fourth or one- 

 tenth part of this amount. I believe Jhat information on these matters 

 is not wanting : a single firm in Liverpool have " corrected " the compasses 

 in several hundred iron ships, and they cannot fail to have received notifi- 

 cation of any such changes as those mentioned above. 



Before dismissing this subject, I will advert to two sources of error, not 

 essential to rny method of correcting the compass, but to which it may be 

 liable if due care is not exercised. The first is, that captains are hardly 

 aware that a very trifling disturbance in the position of the compass (for 

 instance, a change of a quarter of an inch in the height) may very greatly 

 disturb the neutralizing influence of the magnets. The second is, that the 

 artists who correct the compasses are too much inclined to place the cor- 

 recting magnets in the position called " end-on." In this position, the 

 magnet exerts greater deflective power, but it also introduces a force per- 

 pendicular to the ship's deck ; and this force, when the ship heels, 

 produces an imcorreeted horizontal disturbance. While the building in 



